The effects of long-term exposure of arsenic on the ability of hepatic enzymes to convert pro-mutagens to mutagens were tested in both rats and mice. The S-9 fractions isolated from these animals were tested in the Salmonella activation test (strain TA-98). The following compounds were used: 2-acetylaminofluorene, 2-aminoanthracene, 3-methycholanthrene and benzo(a)pyrene. The results indicate that the mutagenicity of some, but not all, chemicals is depressed in arsenic-exposed animals.